The IEEE 802.15.4 standard (IEEE standard for information technology-telecommunications and information exchange between systems—local and metropolitan area networks—specific requirements part 15.4: Wireless medium access control (MAC) and physical layer (PHY) specifications for low-rate wireless personal area networks (WPANS),” IEEE Std 802.15.4-2006 (Revision of IEEE Std 802.15.4-2003), 2006) targets low data rate wireless networks with extensive battery life and very low complexity. Its physical layer is based on a narrowband radio, operating in the unlicensed ISM band at 2.4 GHz. IEEE 802.15.4a (IEEE P802.15.4a (amendment of IEEE std 802.15.4), part 15.4: Wireless medium access control (MAC) and physical layer (PHY) specifications for low-rate wireless personal area networks (LRWPANs),” September 2006.) is an amendment to the 802.15.4 specification. It adds an impulse-radio ultra-wide band (IR-UWB) physical layer operating in several bands of 500 MHz (and 1.5 GHz) from approximately 3 GHz to 10 GHz. This physical layer should offer a better robustness against interference and multipath propagation channels, a higher data rate and the possibility to perform ranging between devices. The IEEE 802.15.4a amendment allows for implementing either a coherent receiver or a non-coherent receiver. Due to their relatively low complexity, non-coherent receivers based on energy detection are of great interest for sensor network applications where devices should be inexpensive and have extremely low power consumption. Generally, non-coherent receivers based on energy detection are however less robust to interference than coherent receivers. It has been shown in [1] that classical energy detection receivers designed to cope with thermal noise only perform close to worst case in the presence of multi-user interference (MUI). MUI occurs due to concurrent packet transmissions. The mandatory medium access control (MAC) protocol in the IEEE 802.15.4a amendment is Aloha. With such a MAC protocol concurrent transmissions inevitably occur. MUI also occurs if several uncoordinated piconets operate in close vicinity, which is a likely scenario for devices operating in unlicensed UWB spectrum.
Scenarios like the ones described above show that there is a need for low-complexity non-coherent receivers that are able to cope not only with thermal noise and hostile channel conditions but also with the presence of MUI created by concurrently transmitting devices.